Intrinsic fragment spins generated in the reactions of 20Ne with 197Au and 238U at 12.6 MeV/nucleon

The average magnitude and alignment of the intrinsic spin of the heavy partner from the reaction of 252 MeV ²⁰Ne with ¹⁹⁷Au and ²³⁸U were determined as a function of Q-value. These spin values were extracted from sequential fission angular distributions obtained in coincidence with projectile-like products. For all Q-values a large out-of-plane anisotropy was observed, while for large negative Q-values an in-plane anisotropy was observed. A very large entrance-channel mass-asymmetry was chosen to provide a stringent test of equilibrium statistical model predictions for the spin alignment. The importance of determining the direction of the line-of-centers of the dinuclear system at scission is discussed. Large values of P_ZZ were deduced for all Q- values. P_XY was observed to be positive in the quasielastic region and negative in the deep-inelastic region. The extracted alignment data are compared to equilibrium statistical model calculations.     

Evidence for spin fluctuations in the deep-inelastic reaction 165Ho + 165Ho at 8.5 MeVamu

Both the magnitude and alignment of the transferred angular momentum in the reaction ¹⁶⁵Ho + ¹⁶⁵Ho have been measured as a function of Q-value via continuum γ-ray multiplicity and anisotropy techniques. The spin transfer and the continuum γ-ray anisotropy increase throughout the quasi-elastic region. The spin transfer as a function of Q-value saturates at ~ 35?/fragment, the anisotropy peaks at a value of ~2 and then decreases to near unity for the largest Q-values. The observed anisotropies are in good agreement with predictions from an equilibrium statistical model in which thermal excitation of angular-momentum-bearing collective modes and neutron evaporation give rise to in-plane components of the angular momentum.     

Investigation of the reaction 208Pb(18O, f): Fragment spins and phenomenological analysis of the angular anisotropy of fission fragments

The average multiplicity of gamma rays emitted by fragments originating from the fission of ²²⁶Th nuclei formed via a complete fusion of ¹⁸O and ²⁰⁸Pb nuclei at laboratory energies of ¹⁸O projectile ions in the range E _lab = 78–198.5 MeV is measured and analyzed. The total spins of fission fragments are found and used in an empirical analysis of the energy dependence of the anisotropy of these fragments under the assumption that their angular distributions are formed in the vicinity of the scission point. The average temperature of compound nuclei at the scission point and their average angular momenta in the entrance channel are found for this analysis. Also, the moments of inertia are calculated for this purpose for the chain of fissile thorium nuclei at the scission point. All of these parameters are determined at the scission point by means of three-dimensional dynamical calculations based on Langevin equations. A strong alignment of fragment spins is assumed in analyzing the anisotropy in question. In that case, the energy dependence of the anisotropy of fission fragments is faithfully reproduced at energies in excess of the Coulomb barrier (E _c.m. ? E _B ≥ 30 MeV). It is assumed that, as the excitation energy and the angular momentum of a fissile nucleus are increased, the region where the angular distributions of fragments are formed is gradually shifted from the region of nuclear deformations in the vicinity of the saddle point to the region of nuclear deformations in the vicinity of the scission point, the total angular momentum of the nucleus undergoing fission being split into the orbital component, which is responsible for the anisotropy of fragments, and the spin component. This conclusion can be qualitatively explained on the basis of linear-response theory.     

The reaction238U +238U at 7.42MeV/u

One-particle-inclusive measurements have been performed for the charge, kinetic energy and angular distributions of reaction products from²³⁸U +²³⁸U at 1 766MeV (7.42MeV/u) incident energy. The deep inelastic products exhibit features similar to those seen in reactions induced by medium heavy nuclei: increasing particle transfer is observed with increasing energy damping, the angular distributions are peaked near the grazing angle, they broaden significantly with increasing energy loss and/or charge transfer. The dominant reaction mechanism, however, is found to be sequential fission of one or both primary reaction products. The reconstructed primaryZ- andQ-value distributions show more particle transfer at a given energy loss than in other systems, i.e. the diffusion process seems to proceed colder in this system. This is confirmed by relatively large cross sections for surviving deep inelastic reaction products belowZ=92. A direct search forα-decay or fission of superheavy nuclei being produced in a deep inelastic reaction and being implanted in a surface barrier detector resulted in an upper cross section limit of 2 ×10^?32cm².     

A study of charge,energy and angular momentum transfer in the 56Fe + 197Au and 56Fe + 107, 109Ag reactions at 7.2 and 8.3 MeV/nucleon

Kinetic energy spectra, charge and angular distributions have been measured for thirty elements produced in the reactions of 401 and 460 MeV ⁵⁶Fe + ¹⁹⁷Au and in the reaction of 470 MeV ⁵⁶Fe + ^{107, 109}Ag. In addition, γ-ray multiplicities were measured at the 470 MeV bombarding energy for both targets at a limited number of angles. The charge distributions for the deep-inelastic component of these systems increase monotonically with atomic number in the measured angular range, whereas, those for the quasielastic component are skewed toward Z-values below the projectile. The angular distributions for the Fe-induced reactions show a smooth evolution from a side-peaked to forward-peaked distributions with increasing mass transfer. This side peak is more intense and more persistent for mass transfers from the projectile to the target. In the quasielastic region the γ-ray multiplicity is observed to increase almost linearly with decreasing Q-value whereas for large negative β-values it is essentially constant and independent of the exit channel mass asymmetry. Finally, angular distributions, angle-integrated charge distributions and γ-ray multiplicities have been compared with a diffusion model in which the dynamics of shape evolution, N/Z equilibration, angular momentum and energy exchange occur via one-body forces.     

Fission product energies and anisotropies following complete fusion of16O+238U

Radioactive recoil techniques have been developed for measuring angular distributions and range distributions of individual fission products following heavy ion induced fission. From these measurements, values can be extracted for the recoil velocity of the fissioning nucleus, the velocity imparted by fission, and the fission anisotropy. These techniques were applied to reactions of 101 MeV¹⁶O on²³⁸U, and confirmed that the reaction mechanism is essentially entirely complete fusion-fission. Accepting this, the data determine the kinetic energy release in forming the various products to a precision of 1%; while the overall magnitude of the energy is in good agreement with previous results, the data suggest a systematic correlation between kinetic energy and the position of a product on the nuclear charge dispersion curve, not previously reported, which is similar to but significantly larger in magnitude than the effect expected from simple Coulomb repulsion. Significant variations in anisotropy are also observed between products, which appear to be partially correlated with the variations in kinetic energy.     

Dynamic picture of the formation of spins of induced fission fragments

A way of calculating the average spins of induced fission fragments is developed, based on the dynamic model of their angular distributions. The range of relaxation times for the degree of freedom associated with the orientation of the axis of symmetry of a fissioning nucleus relative to its total angular momentum is determined by analyzing experimental data on the energy dependences of average spins and the anisotropy of the angular distributions of fission fragments for the ¹²C, ¹⁶O + ²³²Th reactions at E _сm = 55–150 MeV.     

Dynamical model of fission fragment angular distributions

A dynamical model of fission fragment angular distributions is developed. The experimental data on the angular anisotropy of fission fragments is analyzed for the ¹⁶O + ²⁰⁸Pb, ²³²Th, ²³⁸U, and ²⁴⁸Cm reactions at energies of the incident ¹⁶O ions ranging from 90 to 160 MeV. This analysis allows us to extract the relaxation time for the tilting mode. It was also demonstrated that the angular distributions are sensitive to the deformation dependence of the nuclear friction.

     

Q- and Z-dependence of angular momentum transfer in deeply inelastic collisions of 86Kr with 209Bi

The dependence of the in-plane and out-of-plane angular correlations of fragments from fissioning heavy products on the kinetic energy and Z of the light reaction partner have been measured. From the dependence of the angular correlations on Q-value and hence energy loss, together with existing data from which the total angle-integrated cross section as a function of energy loss can be extracted, we have determined the dependence of the angular momentum transferred to the heavy product on the initial orbital angular momentum or impact parameter. The resulting dependence is qualitatively consistent with the sticking limit for a reaction intermediate of touching deformed fragments. More specific nuclear models generally underestimate the angular momentum transfer, although the one-body proximity-friction model accounts for the major fraction of the angular momentum transfer. A recent model incorporating both one-body proximity friction and collective excitations accounts quite well for the observed angular momentum transfer. The Z-dependendence of the anisotropy shows the importance of angular momentum fractionation for the less probable events, where the Z of the fissioning system is appreciably less than that of the target. The transferred angular momentum is shown to be fairly strongly aligned along the perpendicular to the reaction plane, with alignment values of 0.6 to 0.8. The component of angular momentum not along the perpendicular to the reaction plane is found to be primarily oriented perpendicular rather than parallel to the recoil direction. The absolute fission probabilities are found to be qualitatively consistent with J-dependent calculations using the J-values deduced from the angular correlations.     

Angular distributions and anisotropy of the fragments from neutron-induced fission of <Superscript>233</Superscript>U and <Superscript>209</Superscript>Bi in the intermediate energy range of 1–200 MeV

New results of the neutron-induced fission experiments carried out at the neutron time-of-flight spectrometer GNEIS of the PNPI are given. Angular distributions of fission fragments from the neutron-induced fission of ²³³U and ²⁰⁹Bi nuclei have been measured in the energy range 1–200 MeV using position sensitive multiwire proportional counters as fission fragment detector. The recent improvements of the measurement and data processing procedures are described. The data on anisotropy of fission fragments deduced from the measured angular distributions are presented in comparison with the experimental data of other authors.     

Fragment angular distributions in electron-induced fission of 232Th

Fission fragment angular distributions have been observed in electron-induced fission of ²³²Th for electron energies 8.7 MeV ≦ E_e ≦ 30 MeV. For low energies (though above the fission threshold) the angular distributions contain both a dipole and a quadrupole component. The (90°/0°) anisotropy decreases rapidly for higher electron energies but reveals smaller peaks after the onset of second-, third- and fourth-chance fission suggesting that the effective fission barriers for ²³¹Th and ²²⁹Th in second- and fourth-chance fission, respectively, are both characterized by $\text{K =}\text{1}\text{2}$.     

Fragment angular distributions from fission of238U,209Bi and197Au by 140-MeV4He Ions

The fission fragment angular distributions from reactions of 140-MeV⁴He ions with²³⁸U,²⁰⁹Bi and¹⁹⁷Au have been studied. From the anisotropies in the angular distributions, values for? _eff, the effective moment of inertia at the fission saddle point, have been estimated and compared with results obtained at lower bombarding energies. The derivation of? _eff values has included corrections for the effects of incomplete fusion mechanisms on the orbital angular momentum distribution of the fissioning nuclei and for neutron evaporation prior to fission. The results are also compared with heavy-ion-induced fission data for systems of similar fissility. Also, examination of the forward-backward symmetry of the²³⁸U angular distribution substantiates other results which show that the fraction of fission reactions which follow complete fusion of the target and projectile is less than 0.5 for 140-MeV⁴He-ion bombardment of²³⁸U.     

Asymmetric mass distribution from the fusion-fission reaction40Ar+243Am

The mass distributions and total c.m. kinetic energies of fission fragments formed in the reaction⁴⁰Ar+²⁴³Am at bombarding energies of 214, 222, 240 and 300 MeV have been measured using the angular correlation method. Angular distributions and anisotropy for 222 and 300 MeV have also been obtained. A symmetric mass distribution corresponding to the decay of a highly excited compound nucleus was obtained at 300 MeV bombarding energy. However, with decreasing bombarding energy the fission fragment mass distribution becomes asymmetric, the most probable heavy fragment mass being about 200–210 amu.     

A fast-fission component with small mass drift in the reaction84Kr+27Al atE lab=5.9 MeV/u

All reaction products in the range from target- and projectile-like to fission-like fragments were measured for the system⁸⁴Kr+²⁷Al at 5.9 MeV/u beam energy. They are assigned to the various reaction mechanisms on the basis of experimental signatures (energy dissipation, mass and angular distribution). The sum of the measured partial cross sections, including the evaporation residue yield obtained previously, agrees with the total reaction cross section derived from elastic scattering. A small fast-fission component was found, discernible from deep-inelastic reactions by its 1/sin θ angular distribution, and distinguished from compound-nucleus fission by an incomplete mass asymmetry relaxation.     

Proton transfer in quasi-elastic and deep-inelastic reactions in the systems86Kr on88Sr,90Zr and92Mo at energies close to the coulomb barrier

Reaction products corresponding to the transfer of one and several protons have been measured over a large angular range for incident energies of 380 MeV and 400 MeV in reactions of⁸⁶Kr with⁸⁸Sr,⁹⁰Zr and⁹²Mo. For transitions with smallQ-values (total kinetic energy loss TKEL≦10 MeV) the transfer probabilities are deduced. The magnitudes and slopes of these probabilities as function of the distance of closest approach between two nuclei are discussed. The results for single proton transfer are well described by tunneling, whereas the transfer of two and more nucleons into low lying states of the final nuclei seems to be influenced by intermediate transfer steps with larger TKEL. The data give the possibility to discuss the relation between deep-inelastic and quasi-elastic processes. The deep-inelastic data are analyzed successfully by including deformations, charge transfer and statistical fluctuations into the frictional model of Gross and Kalinowski.     

32S-induced reactions at 10 MeV/u bombarding energy

The deep-inelastic processes of the reactions ³²S + ²⁸Si, ^natS, ⁴⁰Ca, ⁵⁸Ni, ⁷⁴Ge are studied at 10 MeV/u bombarding energy employing a kinematical coincidence spectrometer. From the measured energies, momenta, masses and atomic numbers of two heavy fragments the corresponding parameters for the unobserved reaction products and the reaction Q-values are deduced. It is found that the reactions generally show the pattern of a normal deep-inelastic process which is followed by the evaporation of several light particles. But with much less intensities other processes also seem to occur: three-fragment exit channels and incomplete energy damping which is correlated with the emission of a few light particles of high momenta.     

Empfehlungen zum Gebrauch einheitlicher Halbwertzeiten einiger wichtiger Radionuklide

The present work represents the results of angular distribution measurements performed for promt gamma-rays in the range of 1.1 … 1.2. MeV due to the ²³³U(n, f) and ²³⁹Pu(n,f) reactions. The measurements yielded anisotropy values found to be consistent with values previously measured and using the same technique at different gamma energy bands. Such consistency, also found to exist between anisotropy values previously measured for the ²³⁵U(n,f) reaction, is a strong evidence about the energy independence of the anisotropy and indicates that the gamma spectra (at 180° and 90° to the fission direction) are essentially the same. The average values of the fragment angular momentum were calculated according to Strutinsky and Nix-Swiatecki theories. It was found that the values of the average angular momentum calculated for ²³⁴U, ²³⁶U and ²⁴⁰Pu according to Strutinsky,s formula (at different gamma energy bands) are consistent and yield average values which are in good agreement with those obtained from direct measurements.     

An investigation of the properties of single-particle states in the second minimum of 237Pu

In connection with the recent measurement of the magnetic moments of the fission isomeric states in ²³⁷Pu, we investigate the following subjects: (i) The spin alignment of the lowest-lying states in a potential minimum which are populated by (α, 2nγ) reactions, and the fragment angular distributions in fission from isomeric states. (ii) The single-particle level scheme in a very deformed Woods-Saxon potential. (iii) The spin polarizability as a function of the quadrupole deformation.     

Photodissociation of iodocyclohexane at 266 and 277 nm: Energy partitioning and curve crossing analysis

The photodissociation dynamics of iodocyclohexane (C₆H₁₁I) at 266 and 277 nm has been investigated by ion velocity imaging technique. The velocity distributions, angular distributions and relative quantum yields are obtained for I (²P_3/2) (denoted I) and I (²P_1/2) (denoted I*) fragments. The energy partitioning shows that about 70% of the available energy goes into the internal excitation of the photofragments for both dissociation channels. From the angular distributions, we found the value of the anisotropy parameter β for I* at the corresponding excitation wavelength was less than that for I. Based on the measured angular distributions and relative quantum yields, the relative fractions of each excited state to the products are determined. The curve crossing probabilities between the ³Q₀ and ¹Q₁ states are determined 0.503 at 266 nm and 0.443 at 277 nm.     

Gamma-ray spectroscopy with incomplete fusion reactions for the systems 16O +58,64Ni at 100 Me V

Energy spectra and angular distributions of ejectiles with 3 ⩽ Z ⩽ 7 have been measured for the reaction ¹⁶O on ⁶⁴Ni at 100MeV incident energy. Measured optimum Q-values and cross sections are accounted for by an incomplete fusion model. Coincidences between ejectiles detected at grazing angle and discrete γ-rays have been measured for the reactions 100 MeV ¹⁶O+^58,64Ni. The in-plane to out-of-plane anisotropies of discrete γ-rays show a high degree of spin polarization of incomplete fusion residues which can be exploited for spectroscopic studies.